1. Field of the Invention
The present invention relates to a method of detecting a disconnection and a power discontinuity of an external signal input/output unit connected to a machine tool. More specifically, the present invention relates to a method of detecting a disconnection and a power discontinuity of one of master-slave I/O units included in a numerical controller and a machine tool.
2. Description of the Related Art
There is known a configuration in which a plurality of external signal input/output units (hereinafter referred to as “I/O units”) is connected to one another so as to input or output DI/DO signals (input signals/output signals) between a numerical controller and a machine tool. Generally, the numerical controller and the I/O unit communicate DI/DO data with each other.
Furthermore, the numerical controller and the I/O unit communicate states of (that is, alarm information on) the respective I/O units simultaneously with the DI/DO data to use clarification of cause for occurrence of an alarm (see FIGS. 8A and 8B).
A technique for notifying the numerical controller and the machine tool of occurrence of power discontinuity to one of the I/O units as alarm information is already known as disclosed in, for example, Japanese Patent Application Laid-Open No. 2003-209590. However, according to this technique disclosed in the above patent document, it is necessary to hold a power supply voltage for internal circuits of each of the I/O units until the right of a communication line is guaranteed, however, for holding the power supply voltage, it is disadvantageously required to provide a large-capacity capacitor in each I/O unit, resulting in cost hike.
Moreover, a technique for detecting occurrence of a data communication failure at the time of transferring data between a higher-level device and a lower-level device in an information transfer processing apparatus in a telephone exchange system or the like is disclosed in, for example, Japanese Patent Application Laid-Open No. 5-211560. The technique disclosed in the above patent document adopts an auto-scanning scheme. If no reply is received, a communication failure alarm is produced. However, the problem with the auto-scanning-based technique is that the communication failure such as the power discontinuity and the disconnection of a communication line of I/O unit cannot be discriminated from each other (that is, it is impossible to identify whether the failure is the power discontinuity or the disconnection of the communication line during detection of the failure).